Polyaniline is one of the most useful conducting polymers due to its facile synthesis, environmental stability and simple acid/base doping/dedoping chemistry (MacDiamid, A. G. “Synthetic Metals: A Novel Role For Organic Polymers”, Angew. Chem. Int. Ed. 40, 2581 (2001). Since its electrical conducting mechanism was explored in the 1980's, this electroactive polymer has been extensively investigated for many applications including antistatic and anticorrosion coatings, chemical sensors and electrodes for light-emitting diodes, capacitors and batteries. One of the simplest methods for synthesizing bulk polyaniline is the chemical polymerization of aniline with an oxidant ammonium peroxydisulfate in an acidic aqueous solution. Since polymerization is an exothermic process, it has long been recommended that this reaction be carried out at low temperatures with one reactant slowly added into the other under vigorous stirring (Cao, Y., Andreatta, A., Heeger, A. J., Smith, P., “Influence Of Chemical Polymerization Conditions On The Properties Of Polyaniline”, Polymer 30, 2305 (1989); Chiang, J.-C, MacDiamid, A. G, “Polyaniline-Protonic Acid Doping Of The Emeraldine Form To The Metallic Regime, Synth. Met. 13, 193 (1986)). However, when polyaniline is synthesized at room temperature or below via the conventional method, using an ordinary magnetic stirrer bar to agitate the reaction mixture, the product settles out quickly from the reaction solution and films cast from its suspension are rough and discontinuous and therefore unsatisfactory for most applications (FIG. 1). A close look at the purified powder with a scanning electron microscope SEM indicates that as-synthesized polyaniline consists of coral-like, granular particulates.
Processability is crucial to many applications of nanostructured materials. However, a major problem in processing these materials is their stability in colloidal suspensions and their tendency to agglomerate. Aggregation is very common in the production and use of many chemical and pharmaceutical products, especially nanoparticles. Aggregation has been conventionally ascribed to the direct mutual attraction between particles via van der Waals forces or chemical bonding. Aggregation is a common, yet complex, phenomenon for small particles. Strategies for preventing aggregation mainly come from conventional colloid science in which particles are coated with foreign capping agents and/or the surface charges are tailored to separate them via electrostatic repulsions (R. J. Hunter, “Foundations of Colloid Science”, Oxford University Press, New York, 1 (1987)).
Shape control of nanoparticles, especially synthesis of one-dimensional nanostructures, has received growing interest in recent years. Research in this field has created many novel nanostructures for a wide range of applications (Peng, X. G. et al, “Shape Control Of CdSe Nanocrystals, Nature, 404, 59 (2000); Puntes, V. F., Krishnan, K. M., Alivisatos A. P., “Colloidal Nanocrystal Shape And Size Control: The Case Of Cobalt”, Science 291, 2115 (2001); Sun, Y. G., Xia, Y., “Shape-Controlled Synthesis Of Gold And Silver Nanoparticles”, Science 298, 2176 (2002); Kim, F., Connor, S., Song, H., Kuykendall, T., Yang, P., “Platonic Gold Nanocrystals”, Angew. Chem. Int. Ed. 43, 3673 (2004); Peng, X. G., “Mechanisms For The Shape-Control And Shape-Evolution Of Colloidal Semiconductor Nanocrystals”, Adv. Mater., 15, 459 (2003); Pileni, M. P., “The Role Of Soft Colloidal Templates In Controlling The Size And Shape Of Inorganic Nanocrystals”, Nat. Mater, 2, 145 (2003); Xia, Y. et al, “One-Dimensional Nanostructures: Synthesis, Characterization, And Applications”, Adv. Mater., 15, 353 (2003); Huynh. W. U., Dittmer, J. J., Alivisatos, A. P. “Hybrid Nanorod Polymer Solar Cells”, Science, 295, 2425 (2002). Rationally mediating the nucleation and growth process has been shown to be the key to controlling the shape and size of nanoparticles. Mechanical stirring is a routine operation in chemical reactions. Since stirring can affect nucleation and aggregation, this factor must be considered when one carries out or attempts to reproduce a synthetic process involving particles. Of particular importance is that shear in a fluid induced by stirring is strongly dependent on the stirring speed, the geometry and size of the reactor and the structure of the stirring impellers. These factors may be especially important when nanoparticle production is scaled up. Due to a lack of understanding of the nucleation behavior of polyaniline and particularly the effects of stirring, the simple idea that the conventional reaction for the synthesis of polyaniline is capable of producing highly dispersible conducting nanofibers has been overlooked for decades.
In order to make dispersible polyaniline nanoparticles, many methods have been developed, such as emulsion and dispersion polymerizations and a large number of surfactants and templates have been tested to improve the processability of this polymer (Stejskal, J., “Colloidal Dispersions Of Conducting Polymers”, J Polym. Mater. 18, 225 (2001); Ames, S. P., Aldissi M, “Novel Colloidal Dispersions Of Polyaniline”, J. Chem. Soc., Chem. Commun, 88 (1989); Cooper, E. C., Vincent, B. “Electrically Conducting Organic Films And Beads On Conducting Latex-Particles, J. Phys. D, 22, 1580 (1989); Osterholm, J. E., Cao, Y., Klavetter, F., Smith, P. “Emulsion Polymerization Of Aniline”, Polymer, 35, 2902 (1994); Barisci, J. N, Innis, P. C, KaneMaguire, L. A. P, Norris, I. D. Wallace, G. G., “Preparation Of chiral Conducting Polymer Colloids, Synth. Met, 84, 181 (1997); Moulton S. E, Innis, P. C, KaneMaguire, L. A. P, Ngamna O, Wallace, G. G. “Polymerisation And Characterisation Of Conducting Polyaniline Nanoparticle Dispersions”, Curr. Appl Phys., 4, 402 (2004); Stejskal, J. et. al. “Polyaniline Dispersions, 8. The Control Of Particle Morphology”, Polymer, 40, 2487 (1999); Zhang, X., Goux, W. L., Manohar, S. K. “Synthesis Of Polyaniline Nanofibers By Nanofiber Seeding” J. Am. Chem. Soc., 126, 4502 (2004); Zhang, X. Y., Manohar, S. K., “Polyaniline Nanofibers: Chemical Synthesis Using Surfactants”, Chem. Commun, 2360 (2004); Zhang, X. Y., Chan-Yu-King, R., Jose, A., Manohar, S. K, “Nanofibers of polyaniline synthesized by interfacial polymerization”. Synth. Met., 145, 23 (2004); Wei, Z. X., Zhang, Z. M., Wan, M X., “Formation Mechanism of Self-Assembled Polyaniline Micro/Nanotubes”, Langmuir, 18, 917 (2002); Chiou, N. R., Epstein, A. J., “Polyaniline Nanofibers Prepared By Dilute Polymerization”, Adv. Mater., 17, 1679 (2005)). Applicants have recently demonstrated that polyaniline nanofibers can be readily obtained by interfacial polymerization (Huang, J., Virji, S., Weiller, B. H., Kaner, P. B. “Polyaniline Nanofibers: Facile Synthesis And Chemical Sensors”, J. Am. Chem. Soc., 125, 314 (2003) or simply by rapidly mixing an aqueous solution of aniline and an oxidant, instead of slow addition of one reactant to the other (Huang, J., Kaner, P. B. “Nanofiber Formation In The Chemical Polymerization Of Aniline: A Mechanistic Study”, Angew Chem. Int. Ed., 43, 5817 (2004). Thick films can be readily fabricated from colloidal dispersions through casting, while monolayers can be created by electrostatic self-assembly. The exceptional processability of these electroactive one-dimensional nanostructures provides significant advantages in both conventional uses of conducting polymers and emerging applications in nanotechnology.
According to the established theory for the stabilization of colloids, both steric repulsion (by using a polymer or surfactant as stabilizer) and electrostatic repulsion (by introducing charge to the particle surfaces) are often utilized to stabilize a colloid. However, most of the processes for preparing conducting polymer colloids so far have been based on steric repulsion with little attention paid to electrostatic stabilization. (T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynoldsd, “Handbook of Conducting Polymers”, Marcel Dekker, New York, 2nd edn., pp. 423-435 (1997); S. P. Armes, M. Aldissi, “Novel Colloidal Dispersions of Polyaniline”, J. Chem. Soc., Chem. Commun., (1989), E. C. Cooper, B. Vincent, “Electrically Conducting Organic Films and Beads Based on Conducting Latex-Particles”, J. Phys. D, 22, 1580 (1989); J. Stejskal, I. Sapurina, “On the Origin of Colloidal Particles in the Dispersion Polymerization of Aniline”, J. Colloid Interface Sci., 274, 489 (2004); P. R. Somani, “Synthesis and Characterization of Polyaniline Dispersions”, Mater. Chem. Phys., 77, 8.1 (2002); M. Gill, J. Mykytiuk, S. P. Armes, J. L. Edwards, T. Yeates, P. J. Moreland, C. Mollett, “Novel Colloidal Polyaniline Silica Composites”, J. Chem. Soc., Chem. Commun., 108 (1992)) The backbone of the emeraldine form of polyaniline doped by a protonic acid (HA-) is positively charged;
Therefore, a stable colloid can be formed through electrostatic repulsion without using steric stabilizers if the particle size is kept sufficiently small.
One of the major goals in the field of conducting polymers since its inception has been to make them processable. Enormous effort has been directed towards functionalization, copolymerization and blending to enhance solubility. However, there is a trade-off in terms of cost, purity, scalability and conductivity. For example, the solubility of polyaniline can be enhanced through chemical modifications, e.g. inserting substituents either on the phenyl ring or on the nitrogen. (J. Yue, Z. H. Wang, K. R. Cromack, A. J. Epstein, A. G. MacDiamid, “Effect of Sulfonic-Acid Group on Polyaniline Backbone”, J. Am. Chem. Soc., 113, 2665 (1991); H. S. O. Chan, P. K. H. Ho, S. C. Ng, B. T. G. Tan, K. L. Tan, “A New Water-Soluble, Self-Doping Conducting Polyaniline from Poly(o-aminobenzylphosphonic acid) and Its Sodium Salts: Synthesis and Characterization”, J. Am. Chem. Soc., 117, 8517 (1995); Y. Cao, P. Smith, A. J. Heeger, “Counterion Induced Processibility Of Conducting Polyaniline And Of Conducting Polyblends Of Polyaniline In Bulk Polymers”, Synth. Met., 48, 91 (1992)) However, the resulting chain torsion generally causes significant decreases in conductivity. Solubility can also be improved with the use of compatible doping acids, but the counterion-induced processability appears to be only suitable with organic solvents.
Another strategy to address the processability of conducting polymers is to form colloidal dispersions through emulsion polymerization of aniline in the presence of steric stabilizers, such as surfactants, water-soluble polymers or silica colloids. (T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynolds, “Handbook of Conducting Polymers”, Marcel Dekker, New York, 2nd edn., pp. 423-435 (1997); S. P. Armes, M. Aldissi, “Novel Colloidal Dispersions of Polyaniline”, J. Chem. Soc., Chem. Commun., (1989), E. C. Cooper, B. Vincent, “Electrically Conducting Organic Films and Beads Based on Conducting Latex-Particles”, J. Phys. D, 22, 1580 (1989); J. Stejskal, I. Sapurina, “On the Origin of Colloidal Particles in the Dispersion Polymerization of Aniline”, J. Colloid Interface Sci., 274, 489 (2004); P. R. Somani, “Synthesis and Characterization of Polyaniline Dispersions”, Mater. Chem. Phys., 77, 8.1 (2002); M. Gill, J. Mykytiuk, S. P. Armes, J. L. Edwards, T. Yeates, P. J. Moreland, C. Mollett, “Novel Colloidal Polyaniline Silica Composites”, J. Chem. Soc., Chem. Commun., 108 (1992)) However, these stabilizer-assisted dispersions are actually mixtures of polyaniline and other polymers/surfactants, which is disadvantageous for many applications. It is also tedious or impractical to remove by-products from the resultant mixtures. Additionally, the fabrication of nanoscale films from stabilizer-assisted dispersions has proven to be difficult. It has also been found that the quality of the products such as the shape and stability of their dispersions varied with different synthetic batches.